Bias tires having relatively narrower sectional widths had been predominantly used in vehicles until around 1960 because vehicles in those days were relatively lightweight, and the speed demanded for the vehicles were lower, resulting a less burden imposed on the tires. However, radial tires are predominant these days and those having larger widths and assuming flat shapes are particularly demanded due to increase in weight and speed of vehicles (see, for example, PTL 1).
However, the larger tire width sacrifices the vehicle space and thus will degrade comfortability. Further, the larger tire widths increase air resistance and cause another problem of poor fuel consumption, despite the fact that there has been an increasing demand for lower fuel consumption along with increased interest in environmental issues.
In particular, electric vehicles, which are being developed for use in future, need to ensure a sufficient space for accommodating driving components such as a motor for controlling the torque for rotating tires around drive shafts. Ensuring a sufficient space around the tire is also becoming increasingly important in this connection.
Further, the aforementioned wide-width flat-shaped tire has a treading surface with a wide width, which makes it difficult to laterally discharge water toward both sides during driving in rain, as illustrated in FIG. 1A schematically showing the flow line of the water by an arrow, which leads to poor drainage. Further, the wide-width flat-shaped tire has a small contact length L, which is likely to cause a so-called hydroplaning phenomenon in which, as illustrated in FIG. 1A, the tread surface is uplifted due to the water film entered from the treading surface, with the result that the actual contact area is reduced to lose grip, causing a problem of degradation in wet performance.
In view of the above, particularly in a wide-width flat-shaped radial tire, it has been hitherto necessary to dispose, in the tread surface, a main groove that has a large sectional area and extends in the tread circumferential direction.
However, when disposing a main groove having a large groove depth, it is necessary to increase the thickness of the tread correspondingly to the large depth of the main groove, which causes a problem of increase in tire weight and deterioration in driving performance. Further, providing the main groove having a large groove width incurs an increase in negative ratio, which reduces a contact area, causing a problem of deterioration in grip force, that is, deterioration in driving stability and braking performance on a dry road surface, and further, degradation in wear resistance and noise performance.
Further, it is known to be effective to use, in a wide-width flat-shaped radial tire, tread rubber of a small hysteresis loss for the purpose of reducing rolling resistance so as to attain low fuel consumption. However, the use of rubber of a small hysteresis loss incurs a problem of impaired grip performance on a wet road surface.